Going to a higher altitude or reduced oxygen environments is safe when done properly. Millions of air travelers experience high altitude when they fly in aircraft pressurized to 6–8,000 feet. Hundreds of thousands of tourists visit Colorado's high country and stay at altitudes ranging from 8,000 feet (Vail or Aspen, Colo., USA) to 11,000 feet (Leadville, Colo., USA). These same tourists enjoy shorter stays at 12,000 feet (top of Loveland Pass) to 14,000 feet (top of Pikes Peak).
However, medical problems due to high altitude include a number of uncomfortable symptoms and some potentially dangerous conditions, all resulting from the decrease in the oxygen concentration in the blood. Altitude sickness is not a specific disease but is a term applied to a group of rather widely varying symptoms caused by altitude. The primary cause is decreased oxygen. People react differently to altitude at different times and different people react differently to altitude. Physical fitness does not confer any protection against acute mountain sickness and does not facilitate acclimatization. Altitude effects result from the lower oxygen content of the air—not from the lower barometric pressure. At 18,000 feet the amount of oxygen molecules per cubic foot of air is approximately one half that of sea level.
Additionally, going too high too fast causes altitude sickness. When a person is exposed to a higher altitude for longer periods, he/she acclimatizes to the higher altitude. By acclimatizing slowly, a person can usually avoid the symptoms of altitude sickness. Symptoms of altitude sickness may include: nausea, headaches, sleeplessness, weakness, malaise, difficulty breathing, feeling “hung over”, lethargy, a loss of appetite, altered thinking, and/or feeling “intoxicated”.
During acclimatization there is an increase the body's efficiency in absorbing, transporting, delivering and utilizing oxygen. The most important processes in acclimatization are:                (a) An increase in respiratory rate and volume. This change usually begins at around 3,000 feet and may not reach a constant value until several days after arrival at high altitude.        (b) Changes in the pulmonary circulation. During exposure to any kind of low oxygen environment, including high altitude, the pressure in the pulmonary arteries is elevated and the capillaries of the lung are more fully infused with blood increasing the capacity of the circulatory bed of the lung to absorb oxygen.        (c) An increase in the number of red blood cells. Shortly after arrival at high altitude an increase in the number of red blood cells in the blood occurs. Later red blood cell production by the bone marrow is increased so that the blood contains more red cells than at sea level. Since the red cells carry oxygen the increased number of red cells permits each unit of blood to carry more oxygen. This process reaches its maximum in about six weeks.        (d) Increased cardiac output. During the first few days at high altitude, the volume of blood pumped by the heart per minute is increased, which increases the rate of oxygen delivery to the tissues.        (e) Changes in the tissues of the body. Prolonged exposure to altitude is accompanied by the changes in the tissues that use oxygen, particularly muscle, which permit normal function at very low oxygen pressures. These changes include an increase in the number of capillaries within the tissue, and an increase in the concentration of enzymes, which extract oxygen from hemoglobin, as well as an increase in the volume of mitochondria, which are the cellular structures within which these enzymes are located.        
The physiological effects of altitude acclimatization have been documented for many years. These effects include:                (a) An increase in total blood volume        (b) An increase in red blood cell mass        (c) An increase in VO2 max—the maximum amount of oxygen the body can convert to work        (d) An increase in hematocrit, the ratio of red blood cells to total blood volume        (e) An increase in the lungs ability to exchange gases efficiently        
Together these changes produce an increase in the oxygen carrying capacity of the blood and the body's ability to use the oxygen transported resulting in a major difference in the body's ability to perform work both at altitude and at sea level. The net result of such changes is an improvement in athletic performance.
The time required for the different adaptive processes is variable. The respiratory and biochemical changes are typically complete in six to eight days. The increase in the number of red blood cells is about 90 percent of maximum at three weeks. In general, about 80% of adaptation is completed by 10 days and 95% is completed in six weeks. Longer periods of acclimatization result in only minor increases in high altitude performance. However, continued exposure to altitude does maintain the physiological acclimatization. After return to sea level, acclimatization starts to be lost after 10–15 days. Red blood cell counts remain higher for up to 6 weeks.
Living at a high altitude is essential to maximize the oxygen carrying capacity of the blood and improving athletic performance. In their landmark study published in the July 1997 issue of the Journal of Applied Physiology, Dr Benjamin Levine and Dr. James Strey-Gundersen of the University of Texas Southwestern Medical Center demonstrated convincingly that athletes perform best when living (including sleeping) at high altitude and training at low altitude. Their study of 39 elite runners showed a marked increase in performance (at sea level as well as at altitude) among the group that lived at high altitude and drove down to low altitude for training. There was no performance improvement in any of the other groups (living high and training high, living low training low, or living low and training high.).
Further studies have also shown that training at low altitude is critical to getting the best quality training. At high altitude the blood is not fully saturated with oxygen. While the athlete's blood would be 97–98% saturated with oxygen at sea level it may be only 80% saturated at 14,000 feet. As a result the athlete at altitude is unable to work or train as hard. U.S. Olympic Team cyclists at their high altitude training camp found they could work harder by riding cycling ergometers while wearing oxygen masks to simulate sea level. A rider that could put out 400 watts at altitude could put out 480 watts at sea level with the same perceived exertion. In short, athletes benefit more from their training at sea level than from training at high altitude. This study and others show that the optimal training program includes living high and training low.
Research shows that the body's production of erythropoietin (the natural glycoprotein produced by the kidneys that signals the bone marrow to make more red blood cells) goes up dramatically as altitude increases from 6,000 feet (30% increase over sea level) to 14,500 feet (300% increase over sea level.) Most training regimens simply do not train the athlete at low enough elevations while allowing them to sleep at high enough elevations to gain the maximum benefit from training. In a preferred embodiment, it is recommended that a person sleep at an altitude of 8,000–13,000 feet for the maximum acclimatization effect, after a period of acclimatization at lower altitudes.
What limits exercise at high altitude is the lack of oxygen concentration. Mountain air contains less oxygen than air at sea level. By reducing the amount of oxygen in the room the equipment simulates high altitude.
The amount of exercise that can be performed at high altitude is less than at sea level and the heart rate reached during maximal exercise is less. This indicates less cardiac work. Maximal exercise capacity decreases progressively with higher altitudes. So it would be desirable to sleep high and train low.
The beneficial effect of sleeping high and training low is that the oxygen processing capacity of the body is increased. This allows the body to do more work (run, swim, ski, or cycle faster) at the same level of physical exertion and heart rate. The body can also perform the same amount of work as it did prior to living high and training low at lower exertion rates and lower heart rates. The athlete can remain in an aerobic state longer and work harder without becoming anaerobic. The athlete can perform at higher levels while still using fat as a fuel instead of sugars. This allows for greater performance levels and faster times while decreasing lactic acid production.
Research has also shown that athletes who train at low altitude but five at high altitude perform better in endurance, and running speed, than athletes who train and live at high altitude or who live and train at low altitude. “High-low” athletes also recover faster and increase their VO2 max. Moreover, when people plan to participate in an athletic event at high altitude it is desirable to train at high altitude before the event to acclimatize to the conditions. Therefore, there is a need to simulate both high altitudes and low altitudes.
There have been various attempts at providing systems for simulating a different altitude from the altitude that a person resides in order to presumably address the debilitating effects of increased altitude, and/or to obtain some of the advantages of purposely simulating different altitudes for, e.g., athletic training. Some of these are discussed immediately below.
Heiki Rusko in Finland introduced nitrogen into an enclosed house using bottled nitrogen to reduce oxygen levels in an altitude house. This approach suffered from high cost, low convenience and an inability to control CO2. Only high altitude was simulated, not low altitude.
Nils Ottestad in Norway improved upon this concept by using an oxygen concentrator, a magnetic gate, a fan, a CO2 scrubber, oxygen sensors, and CO2 sensors. In his invention, the oxygen concentrator was running at all times. A user activated the CO2 scrubber. Oxygen sensors measured oxygen levels and sent data to a control panel that only controlled the alarm, the magnetic gate, and a fan. This approach suffered from requiring the user to control the CO2 scrubber and a general lack of sophistication. The control panel did not control the oxygen concentrator, the CO2 scrubber, or the high CO2 alarm. Fans were not employed in high CO2 situations. Only high altitude was simulated, not low altitude.
Additionally, U.S. Pat. Nos. 5,964,222 filed Dec. 3, 1997, 5,799,652 filed Jul. 21, 1995, 5,924,419 filed Feb. 8, 1997, and 5,850,833 filed May 22, 1995, all of which have Kotliar as the inventor, describe the use of an oxygen concentrator to introduce nitrogen into an environment to thereby provide oxygen depleted air. This approach suffers from a limited ability to control altitude and CO2 levels. Moreover, Kotliar's systems are only capable of simulating high, rather than low altitudes.
Accordingly, it would be desirable to have a more cost effective method and apparatus that could better simulate variable altitudes, and in particular, easily simulate both lower and higher altitudes than the current altitude of a person.